1. Field of the Invention
This invention relates to the generation and transmission of electrical pulses with desired waveforms, particularly in the context of automatic test equipment (ATE) pin drivers.
2. Description of the Related Art
Pin drivers are commonly used in automated test equipment systems to generate test signals in the form of digital or analog voltages for delivery to the input pins of a device under test (DUT). They may be used, for example, to test memory chips having many input pins. Proper waveshape and timing accuracy of the signal at the DUT-are primary objectives of the overall system design, and are figures of merit which distinguish between different available ATE systems.
Due to increasing system speeds, waveform fidelity has become harder to maintain. While a pin driver itself may be able to produce a pure, undegraded signal, various elements of the overall system have a detrimental effect on waveform fidelity. These include the board on which the pin driver is attached (commonly called the channel card), the relays used to direct the signal, and the connectors which transmit the signal to other boards. The large number of pins on test devices such as memory chips also leads to long signal paths. The net effect is to lower the bandwidth of the test pulse signals that are delivered to the DUT. Although the driver itself may produce crisp, clean pulses with sharp corners, the net effect of the signal path between the pin driver and the DUT is to round off the corners of the test pulses, causing the transitions between the pulse edges and the full scale pulse value to be slower and more gradual than is desired. This is illustrated in FIG. 1, in which the clean pulse 2 generated by the pin driver is indicated by a solid line, with the rounded corners 4 induced by the transmission system indicated by dashed lines.
Attempts to remediate the pulse degradation have typically been directed towards reducing the effects of the transmission system. For example, inductive tuning networks have been provided along the signal path to produce a ringing which compensates for the path's effect on the pulse shape. Another approach involves adding copper stubs to the circuit board with a characteristic impedance that matches that of the transmission elements. Efforts have also been made to develop improved relays whose effect on the final pulse delivered to the DUT is reduced. All of these approaches are directed at modifying the transmission system so as to reduce or compensate for its effects on the clean pulse, and require a significant modification to the channel card.